Process of bleaching kraft pulp with alkaline hypochlorite bleach acidifying the pulp containing residual chlorine and then bleaching with alkaline peroxide



Dec. 23, 1958 w, sc o 2,865,701

PROCESS OF BLEACHING KRAFT PULP WITH ALKALINE HYPOCHLORITE BLEACH ACIDIFYING THE PULP CONTAINING RESIDUAL CHLORINE AND THEN BLEACHING WITH ALKALINE PEROXIDE Filed Dec. 7, 1953 w 80 (D l-Ll E A C ONTROL I 78 (NO ACID) m m O. 2|.l 'l D 74 HYPOOHLORITE ACID PETROXIDE STAGE STAGE STAGES WILLARD F. SCHROEDER INVENTOR.

ATTORNEY United States bleached pulps having RINE AND THEN BLEACHIVG WITH ALKALINE PEROE Willard F. Schroeder, Ashtabula, Ohio, assignor to 'National Distillers and Chemical Corporation, a corporation of Virginia Application December 7, 1953, Serial. No. 396,709 2 Claims. (Cl. 8-106) This invention is concerned with a new process for bleaching of wood pulp which produces a surprising and unexpected bleach response using a preoxide treatment in the last bleaching stage. This new technique is based on a combination of stages employing critical facto'rsand is readily adaptable to and/or incorporated intoexistiug multi-stage bleaching operations. The process yields both superior-brightness stability and strength properties.

Although it has previously been shown that the use of 2 v a peroxide bleach in the last stage of kraft pulp bleaching produced a more brightness-stable pulp than any other bleaching agent, the bleach response of the pulp to peroxide in the last stage was found to be quite poor compared with hypochlorite or chlorine dioxide. Various methods have been proposed to solve this problem. For example, the use of sodium peroxide in the last alkaline extraction stage has been suggested as-a means of improving the final pulp brightness but the brightness stability of the pulp was found to be inferior. Also, the superbleaching of kraft wood pulp with peroxide at high consistency for relatively long time periods has also been advocated as a means of improving bleach response. Because of the lower alkalinity of the formula in this procedure, the brightness stability is not as good as that of the process herein described. However, this technique requires a large storage area for the bleaching 'pulp and expenditures for additional equipment and result-sin high labor costand inflexibility of operation.

A very satisfactory method has been found for improving the bleach response of peroxide and, more particularly, of sodium peroxide in the last bleaching stage where peroxide can be used to the best advantage. This results in a better final brightness of the pulp. and also in a better brightness stability of the bleached pulp. The final pulp strength is also high if proper. critical factors aresatisfactorily controlled.

In using this novel technique in a. typical .four or five stage process, the unbleached kraft pulp is first chlorinated and alkaline extracted'in the usual mariner, or any other known initial sequence of steps can be employed. Following these preliminary steps, a hydrochlorite bleaehing stage is carriedout. The alkalinity of the hypochlorite bleach is adjusted at a high level to give a pH value of about 9.5 i 1.0 pH units in about one-half to two-thirds the normal bleaching time for the alkaline hypochlorite stage, e. g.. one to two hours bleaching'time rather than 1 /2 to 3 hours. The residual chlorine remaining .onthe pulp should be between and 35%, based'on the total amount added. The pulp is preferably then diluted to 3% to 6% consistency and adjusted to 'a pH of'ab'o'ut 5 with a mineral acid, preferably sulfuric acid or hydrochloric acid. After a period of from one'to thirty minutes, the pulp isthen washed in a normal manner atent C "a with this. technique be 2,865,701 Patented Dec. 23, 1958 given at least one final stage bleach using 0.1 to 1.0% sodium peroxide at above F., preferably to F., the pulp having 12% to 16% consistency. It has been found that the final peroxide bleach stage can also be quite satisfactorily carried out on a pulp having a density of less than 10%, for example, 4 to 8%; When the above-described combination and sequenceof steps is used, the final pulp brightness is much higher than when this method is not used, and the pulp has a higher cupriethylenediamine disperse viscosity. The use of this improved technique further results in a brighter pulp having equivalent brightness stability than does an alkaline extraction followed by a bleach in the normal manner with hypochlorite, washing, an alkaline extraction, and an identical last stage sodium peroxide bleach. For the best final results, there should be maximum pulp brightness combined with maximum pulp strength.

The sequence or combination of steps involved in the practice of the invention shows the striking results when applied to chemical wood pulp especially prepared by an alkaline pulping process such as kraft or soda pulp, although the method can, if desired, also be applied to sulfite pulp. It is not considered suitable for use on the mechanical type pulps since this pulp type is not normally bleached with a multi-stage process, but'r'ather a single stage process is used. The large amoufitof hypochlorite required to give the above-described efiect prior to a peroxide bleach would make the process unecofioiiiieal when used on mechanical ty e pulp.

In preliminary preparation of pulp, the customary bleaching operations may be used without significant changes in procedure. Iii-general, the is some type of chlorine treatment.

initial bleaching either a straight chlorine treatment or it may be carried out by first adding chlorine followed by the addition of lime after a few minutes. The nextstep, in either case,

will be an alkaline extraction stage using normal conditions as practiced in the pulp industry. Thus, the pulp is in condition for the sequence of steps which gives the outstanding brightness and pulp strength qualities which are characteristic of this improvement.

The next stage or treatment step, characterized in the examples as the third stage, constitutes a hypochlorite .treatment. In this stage, an alkaline hypochlorite is used,

residual chlorine remaining based on the amount originally added. Compared to the usual conditions of open ation, in order to accomplish this, there should be either a higher level of alkalinity-used and/or the bleach period should be limited to a shorter period of time. The latter procedureis preferable. The pH of the pulp at--the-'end of this step should be about 9.5 $1.0 pH units. Thus, if thezhypochlorite stage is too long or is carried out under conditions which are too drastic, the final brightness ma be very good but the pulp strength will be lowered con siderably. Thus, if the residual chlorine falls below 10%, the hypoehlorite stage has been of too long duration and the'final-pulp strength will be undesirably low.

If theresidual chlorine is too high (above about 35%), this indicates that the hypochlorite treatment has not been strenuous enough to accomplish the necessary bleaching action and thereby give the pulp the required brightness. .It is recommended that the hypochlorite bleaching time about one-half to two-thirds of This step may be' that normally used which is approximately 1 /210 3' hours.

The acid treatment which is an integral part of this invention is'best characterized, not as a separate step, but rather as an additional or supplemental part of the hypochlorite treatment. In carrying out studies to establish the best sequence of treating steps, it, has been found best not to wash the pulp after the hypochlorite bleaching treatment. In other words, rather than washing the pulp. following the hypochlorite treatment, better pulp brightness results are obtained if it is directly and im mediately thereafter treated with acid under controlled conditions.

In carrying out the acid treating stage, a numberof critical factors should be carefully controlled in order to obtain the best results. The acid used should be of the mineral type and is preferably non-reducing. Thus, either sulfuric acid or hydrochloric acid can be used quite satisfactorily but, S0 a reducing acid, has been found to give less satisfactory results. Also, alum, a commonly employed acidifying agent, does not give good results.

The pH of the pulp should be adjusted to a value of approximately 5. A slight variation in pH of from about 4 up to 5.5 can be allowed. If a pH of 6 is used, much poorer results are obtained, and pH values of less than 4 also give mediocre improvement. This variable is very critical and should be carefully controlled in order to achieve the outstanding results of the invention.

With regard to the time of treatment, this is dependent somewhat upon the nature of the various commercial pulp bleaching systems. that prolonged acid treatments are little more effective than relatively shorter periods of from 1 up to minutes.

Although it is not believed to constitute a critical factor in the treatment, it has also been observed that the results given by acid treating a high density pulp of 12 to 16% consistency are somewhat better than those obtained-from treating a lower density pulp of 3 to 6%. However, superior brightness values are obtained in either case and it is not intended to limit the invention to pulp having any particular densities. This is an'advantage since it avoids the necessity of thickening or diluting the pulp prior to using the acid treatment.

The effect of the temperature during the acid treat- However, it has been observed I ment hasbeen found not to be critical. A temperature within the range of 35 to 100 F. can be employed.

Most important to the ultimate success of the invention is the carrying out of the hypochlorite bleach'such that there is a relatively high residual available chlorine. It is only in this way that there is obtained a better response in the final peroxide bleach stage' Following the acid treatment at least one peroxide bleaching treatment is used. It is much preferred to use an alkaline peroxide bleach as the final treatment. The alkaline peroxide solution may be prepared from hydrogen peroxide and substantially any fixed alkali or from the alkaline peroxides such as alkali metal peroxides, alkaline earth peroxides, as well as perborates and the like.

This treatment gives improved brightne'ssand brightness stability. A hypochlorite bleach at this stage has been found not as satisfactory. The initial pH in this stage is about ll .5 pH units. The required amount :of sodium peroxide ranges from 0.1 up to 1%. The peroxide solution also generaly contains 1 to 3% sodium silicate as a buffer and 0.01 to 0.05%"Epsom salts as a stabilizer. The alkaline peroxide bleaching treatment can be carried o-utat' 1l0-to 180 F., and preferably from 140 to 160 F. The consistency preferred for this step is 10 to 20% but low densities of less than 10% final peroxide bleach in more than one stage. Thus, data shows that if the peroxide used in one bleaching stage consistency also give good results in the peroxide quence of steps can be used in conjunction with any pulp process wherein chemical pulp is subjected to multiple bleaching operations. It can be incorporated into the process either subsequent to or prior to other stages for bleaching and other treatments.

The invention will be further illustrated by the following examples. The brightness values shown are measured on a percent reflectance scale in which magnesium oxide is taken as 100%. The brightness values shown were measured on a Hunter refiectometer. The density or consistency is measured on the basis of oven dry pulp weight. The percent residual chlorine values are based on the amount of residual chlorine atthe end of the hypochlorite bleach.

The, kraft wood pulps used in Examples I through IX below were subjected to, various preliminary bleaching stages, each series of which differed slightly. These different kraft pulp samples will be. identified in the following examples as pulps A and B. The individual stage treatments of these pulps are shown in the following outline:

nation of stages is employed in the appropriate sequence. In these experiments pulp Sample A described above was used. In no case was the pulp washed between the hypochlorite treatment and the acid treatment. For the acid treatment stage, the samples were treated at 4% consistency with H to give the final pH shown. The time of the acid treatment stage was approximately 15 minutes and the pulp was washed after the acid treatment, prior to the final peroxide bleach stage. The data obtained is shown in Table 1. From this data it can readily be seen that a higher brightness value is reached and a greater brightness increment is shown for samples'having the acid treatment followed by a peroxide bleach.

Neither of these steps employed independently, however, gives satisfactory brightness values.

The accompanying figure is a graph'of the values of Table 1. This figure clearly summarizes the final outstanding results achieved by the invention. The brightness of thezpulp is plotted according to the sequence of stages as shown in Table 1. Each of the three pulp samples was subjected to different hypochlorite bleach periods giving between 10 and 35% chlorine residual v'aluesan'd'pI-I values of 9511.0 pH units. A control pulp which received no acid treatment is also shown. It should 'be noted that within the preferred chlorine residual values and pH range for the pulp, the final high level of brightness reached after acid treatment andsubsequent peroxide treatment is substantially constant. This .level is wellabove the brightness reached by the control sample which received no acid treatment.

Table 1 Calcium Hypochlorite Bleach Stage Acid Treatment" Na'z02 lea h Hunter Brightness Bright Increment Percent Bleach Final Percent Acid Pulp Percent Percent ness of of Bleach Available Time pH Residual used pH NazO: Residual 1 Pulp 012 Used (hr.) Chlorine N220:

2. 7 1 1o. 35 34.9 I. 70. 5 2. 7 1 10. 35 34. 9 76.0 a 2. 7 1 10. 35 34. 9 v 83. 6 i 13. 1 2.7 1.5 9.6 28.8 I 73.4 1 2. 7 1.5 9. 6 28.8 77.3 V 2. 7 1.5 9.6 28.8 v 84.2 f 2. 7 2.0 8.85 21. 1 7516" 2.7 2.0- 8.285- 21.1 78. 4 228 2 2. 7 2.0 8.85 21.1 79. 3 2.7 2.0 8.85-1- 2L1 84.2 7..6

EXAMPLE -.II the'higher final viscosity with the equivalent final bright- A. similar, relatedseries*ofexperiments was -carried out to study the effects of available chlorine residual 'and pulp alkalinity on brightness:and,.their..- relation to:pulp; viscosities: An alkaline-extracted; pulp having a viscosity; of 19:5 centipoises was-used-for %these -tests; The -pulp- Sample..A was. not.washed..between. the.hypoch1or-ite.

bleaching stage and the acid"tr'eatment stage; Thegdata obtained is shown in Table Z-beIoWu. 7 These data. show that goodsfinal brightness is obtained L when the hypochlorite bleaching step is directly followed by an acid treatment and then a peroxide bleach. However, these data show'thaaiforany particular sequence, the longer the hypochlorite bleaching stage the lower the viscosity. Also, if the shorter hypochlorit'e bleaching period is followedfby an.acid.'treatment, and then a peroxide'bleach, the fifialipnlp brightness is thesam'ej as 40 when a longer hypochlorite'bleachis used; Thus, the preferred conditions include a relatively shorter hypochlorite bleaching stage followed by acid treatment-and-r peroxide bleaching. Also, the higher the pulp pH and? ness that is obtained. Therefore, the preferred conditions of-the hypochlorite bleach stage yield a pulp having a pH 0f 95:1.0 pH units, and from 10% up to 35% residual chlorine.

EXAMPLE III A-set of experiments was carried out to determine the effectlofr'various pH values of the acid treatment. A pulp B" sample was used for these tests. The calcium hypochlorite bleach stage was the same in each case. The percentage of available chlorine used was 2.0%, and thetbleachtimewas sl.5..hours. Following thistreatment, the fihaLpHcfQthe pulp..was.8l5, and the residual chlorine value was.18."6%'. The pulp, samples weresnot'washed prior to the acid treatment stage. ,The results are shown in Table-31 These data show that Ithe.brig'h'tness response to the peroxide stage is' quite good at a pH 'of about'SfO. As the pulp becomes less acidic, the brightness response decreases considerably: As the pH approaches 4.0, the brightness-response-alsodecreases: A pH-value--of-from 410 10.5.5. is preferred and 5.0 appear-s2to5'give maximum percent residual chlorine of the hypoehlorite bleachistep,v 4.5 good results.

T able 2 Calcium Hypochlorite Bleach Stage Acid Treatment NazOz Bleach Hunter Brightness Bright- Increment Percent Bleach Final Percent CED 1 Acid Pulp Percent Percent GET) 1 ness of Bleach Avail. Time pH Resid. Vise. used-- 1 pH-- NazOQ Resid.- Vise" a Cl; Used (l1r.) C12 (cps.) N 1202 (cps) 2.7 1.0 10. 35' 34.9 70.5 2. 7 1.0 10.35 3419 76. 0 2..7 1. 0 10-35. 34. 9 11.9 83. 6 13. 1 2. 7 1. 5 9.6 28.:8 73.4 2.7 1.5 9.6 28.8. 77.3 2.7 1. 5 9. 6- 2858 17.3 8.1 84. 2 2. 7 2.0 8. 85 21.1 75. 6 2.7 2.0 8. 85 21.1 1.1 8. 0 78. 4 2. 8 2. 7 2.0 8.85 21. 1 79. 3 2. 7 2.0 8.85 211.1 15.1 7. 7 84. 2 7.6 2.7 1.0 8.60 31.1 74.2 2. 7 1.0 8. 31. 1 78.8 2. 7 1.0 8. 60 r 31.1 21.6 7.7 83.4 9. 2 2.7 1. 5 8.0 18.3 78.8f 2. 7 1. 5 8.0 18.3 81.0 2. 7 1.5 8. 0- 18. 3 15.1 84.6 5.8 2.7 2.0 1. e5. 11.1 so. 1 2.7 2.0 7. 11.1 0.4 81.3 2.7 2.0" 7. 65.. 11.1 81. 8 2.7- 2.0. 7. 65 11.1. 9.1 84. 7

l TAPPI Standard Method T230sm-50 cuprlethylenediamine disperse viscosity for pulp solution.

Table 3 Acid Treatment NazOz Bleach Hunter Brightness Bright- Increment ness 01 Bleach Acid Percent Pulp Time Pulp Per- Percent of Pulp used ConpH (min) Washed cent Residual slstency Na202 NazOz 73.7 0. 5 78. 8 E 904 4 6. 05 16 Yes. 75. 7 H1804 4 6 05 15 Yes 0. 5 80.3 E1804 4 5. 05 15 Yes 76.1 H1804 4 5.05 15 Yes 81.5 H280 4 4. Yes 76. 1 H1804 4 4 10 15 Yes. 0.5 81.3

EXAMPLE 15 time was 1.5 hours. The final pH was 8.55 and the Another set of experiments was conducted to study the present residual chlorine was 20.8. Data is shown in efiect of the acid treatment time. Table 4 shows the Table 5 below. Pulp consistencies of 4 and 12% gave effects on kraft pulp B. The samples were subjected to in the acid treatment stage substantially equal brightness the same calcium hypochlorite bleaching treatment. increments. Thus, the pulp consistency in the acid treat- About 1.8% available chlorine was used at a bleach time ment may vary over a relatively wide range.

Table 5 Acid Treatment N220: Bleach Hunter Bright- Brightness ncss Increment Acid Percent Pulp Time Pulp Per- Percent of Pulp of Bleach used ConpH (min.) Washed cent Residual slstency NazOr N810:

0.35 3.6 19.1 H1504 4 s. 21 Yes 78.8 H1504 4 5. 35 21 Yes 0. as 9. 1 s2. 6 H2804 12 4.50 30 Yes 19.0 H2804 12 4. 30 Yes 0. as 9.1 as. 2

of 1.5 hours. The final pH was 8.5 and the residual 85 chlorine value was 14.0. Good brightness values are E MPLE VI obtained with acid treatment times of from 5 up to 30 Another series of tests was made to determine the effects minutes. It should be noted that not much increase in of various acids used in the acid treatment. Pulp B was brightness is noted when the time period is increased used for these tests. The hypochlorite bleach treatment from 15 up to 30 minutes. 40 was carried out using 2.0% available chlorine and a bleach Table 4 Acid Treatment N210, Bleach Hunter Bright- Brightness ness Increment Acid Percent Pulp Time Pulp Per- Percent of Pulp of Bleach used ConpH (mid) Washed cent Residual sistency N820: NazO:

76.7 0.35 19.5 E1804 4 5.15 5 Yes 78. 0 E1804 4 5. 15 a Yes 0. as 81. 5 HZSO. 4 5.10 15 Yes 78.3 H1801 4 5. 10 15 Yes 0. 35 21. 4 H1804 4 5. 20 so Yes 19. 2 H1304 4 5 20 30 Yes 0. as 32.1

EXAMPLE V time of 1.5 hours to give afinal pulp pH of 8.5 and 20.2% A study was also made of the efiect of the consistency residual chlorine. The results are shown in Table 6. of the-pulp when subjected to the acid treatment. Kraft These data indicate that, of those tested, H 80 and pulp B was used for these tests. I A calcium hypochlorite HCl are the preferred acids for the acid treatment. Sulfur bleach stage preceded the acid treatment in every case. dioxide appears to be somewhat less satisfactory, and The percent available chlorine used was 1.8 and the bleach alum is considered to be unsatisfactory.

Table 6 Acid Treatment Na oa Bleach Hunter Brightness Brightness Increment Percent 7 Pulp Time Pulp Percent Percent of Pulp 01 Bleach Acid used ConsispH (mm) Washed N310: Residual tency Na O;

. 78. 0 H 4 4. 10 13 Yes 78. 5 H1804 4 4.10 13 Yes 81.8 1101 4 4. 05 15 Yes 78.6 HCl 4 4. 05 15 Yes 82. 0 H18 0; 4 4. 20 17 Yes 77. 8 E2501. 4 4. 20 17 Yes 81. 3

r AMSOA; 4 5.1 16 Yes 74.8

. AMSOQI 4 5. 1 16 7 Yes 80. 5

9 ,7 EXAMPLE VII A series of tests was also'conducted on pulp Sample B toshow that sodium hypochlorite could be used equally as Well as calcium hypochlorite in the hypochlorite bleach EX M X ,1 For this experiment asample of-k'raft woodpulp was prepared by chlorinating about 1000 parts 'byweight of southern pine 'kraft wood pulp at 3.5% consistency and The efiect of washing the pulp before the acid treatstage. The percent available chlorine was 1.6 and the 5 a: a 67 F. temperature with 5.0% available chlorine bleach time was 1.5 hours. A final pH of 9.2 and 23.7% for minutes. The percent res'idualchlorinewas about residual chlorine was found. Data are tabulated in 3.3% and the pulp brightness was 21.6. The pulp was Table 7. then washed and centrifuged and given an alkaline ex- Table 7 Acid Treatment N 8.20: Bleach 4 Hunter Brightness Brtght- Increment Acid Percent Pulp Time Pulp Per- Percent ness Bleach used Con- 1 pH (min) Washed cent Residual of Pulp slstcncy NazOz N -202 72.5 0.35 41.5 76.6 4.1 Has 04 4 5. 05 Yes 74. 2 H2804 4 5.05 15 Yes 0. 35 a7 9 80.8 8.3 H01 4 5.2 15 Yes 74.6 H01 4 5. 2 15 Yes 0. 35 37. 9 s0. 9 s. 4

EXAMPLE VIII traction consisting of a 2% NaOH at 154 vF. for about two :hours. The washed pulp had a Hunter brightnessof 30.1.

A portion of the alkaline extracted wood pulp was bleached with 2.5% available chlorine, as calcium hypochlorite, and 0.85% NaOH at 13% consistency and a temperature of 96-99 F. for 1 /2 hours. The residual chlorine at this time was 28.1% or 0.702% available C1 on dry pulp basis, and the pH was 9.8. For comparison purposes, one portion of the pulp indicated as Sample- E was directly treated with H SO at pH 5.0 and 4% consistency for 15 minutes according to the process of the invention. The remainder of the pulp batch was Table 8 Calcium Hipochlorite Acid Treatment N810: Bleach Bleach Stage Hunter Brightness Bright- Increment Final Residual Pulp Acid Percent Pulp Time Percent Percent ness of of Bleach pH Chlorine Washed used ConsispH (min.) Na O Resid Pulp teney NagO 8.55 20.8 76.3 8.55 20.8 0.35 3.6 79.1 8.55 20.8 Yes E2804 4 5 2 30 76.9 8. 20. 8 Yes H2SO4 4 5. 2 30 0.35 7. 3 80. 6 8.55 .20. 8 No H2304 4 5. 35 27 78. 8 8. 55 20.8 No H2SO4 4 5. 35 27 0.35 9. 7 82. 6

EXAMPLE IX washed and centrifuged. One portion, Sample F, of this washed pulp was adjusted to pH 5.0 by treatment with H at 4% consistency for 15 minutes. Another portion, Sample G, was treated with 0.702% available C1 at pH 5.0 for 15 minutes. Both pulp Samples F and G were then washed and thickened. The remainder of the washed pulp, Sample H, was evaluated for use as a control in measuring the brightness improvement obtained by the various treatment methods.

Table 9 below shows a rsum of the data on the preparationof the pulp. The percent residual chlorine 1 Equivalent to 0.70% available 01: on the dry pulp basis as used on Sample G,

l. j .j, Table 10 Final 'Percent Hunter Bright- Pulp used Pulp pH Residual Brightness Incre- N850: ness meut These data indicate that the controlled acidification of the unwashed hypochlorite bleached pulp gives somewhat better brightness response than washing the pulp and giving an identical hypochlorous acid bleach'with an equivalent amount of chlorine under the same conditions.

What is claimed is:

1. A process for producing a kraft pulp of improved brightness stability and strength which comprises chlorine "bleaching a kratft pulp, alkali extracting said chlorine bleached pulp, bleaching the resulting pulp with an alkaline hypochlorite bleach to provide a pulp of pH 9.5 :1.0 pH unit and containing a residual chlorine: content of from 10 to of the amount of the available chlorine from the alkaline'hypochlorite, acidifying the resulting pulp containing said amount of residual chlorine with a non-reducing strong mineral acid to a pH of 4.0 to 5 .5,

and subjecting the resulting acidified pulp to at leastoue alkaline peroxide bleach to produce a pulp of improved strength and brightness stability.

2. A process, as definedin claim 1, wherein the ,nonreducing strong mineral acid is a member from the group consisting of sulfuric acid and hydrochloric acid.

References Cited in the file of this patent UNITED STATES PATENTS 1,768,821 Bradley July 1,1930 1,771,064 Rue July 22,1930 1,820,502 Schuber Aug. 25,1931 2,203,205 Rawling June 4, 1940 2,668,095 Fennell Feb. 2, 1954 OTHER REFERENCES Aitken: Peroxides to Bleach Sulphate Pulp, Canadian Chem. Processing, February 1953, pp. 42 and 44, 

1. A PROCESS FOR PRODUCING A KRAFT PULP OF IMPROVED BRIGHTNESS STABILITY AND STRENGTH WHICH COMPRISES CHLORINE BLEACHING A KRAFT PULP, ALKALI EXTRACTING SAID CHLORINE BLEACHED PULP, BLEACHING THE RESULTING PULP WITH AN ALKALINE HYPOCHLORITE BLEACH TO PROVIDE A PULP OF PH 9.5=1.0 PH UNIT AND CONTAINING A RESIDUAL CHLORINE CONTENT OF FROM 10 TO 35% OF THE AMOUNT OF THE AVAILABLE CHLORINE FROM THE ALKALINE HYPOCHLORITE, ACIDIFYING THE RESULTING 